Gaseous effluents containing contaminant gases are produced by a variety of operations. For example, sulfur dioxide is generated in various chemical and metallurgical operations, including sulfur-burning sulfuric acid processes, spent sulfuric acid plants, roasting or smelting of sulfidic metal ores, Claus plants, and concentrates and the combustion of sulfur-containing fuels (e.g., flue gases from coal-fired power plants). Carbon fuels play a significant role in the generation of electricity, providing energy for heating and fuels for transportation. Most carbon fuels contain sulfur that when burned turns into sulfur dioxide. The sulfur dioxide emitted contributes to a wide range of environmental and health problems. As the emerging economies expand, their demands for energy rapidly increase and as lower sulfur content carbon fuels are depleted, more and more oil and coal reserves having increasingly higher levels of sulfur will be utilized leading to increased sulfur dioxide emissions.
There are also increasing regulatory pressures to reduce sulfur dioxide emissions around the world. The most commonly used method to remove sulfur dioxide is through absorption or adsorption techniques. One common approach is to contact sulfur dioxide with an aqueous stream containing an inexpensive base. The sulfur dioxide dissolves in water forming sulfurous acid (H2SO3) that in turn reacts with the base to form a salt. Common bases are sodium hydroxide, sodium carbonate and lime (calcium hydroxide, Ca(OH)2). The pH starts at about 9 and is lowered to about 6 after the reaction with sulfur dioxide. A one stage wet scrubbing system usually removes over 95% of the sulfur dioxide. Wet scrubbers and similar dry scrubbing approaches require a capital investment, variable costs due to lime consumption and solids disposal plus the energy consumption and utility consumption used to operate the sulfur dioxide removal system.
Instead of reacting with a base like lime, sulfur dioxide in effluent gases that otherwise may be emitted to the atmosphere may be recovered to be sold as a refined sulfur dioxide product, used as part of the feed gas to a contact sulfuric acid plant and recovered as sulfuric acid and/or oleum to meet the growing global demand of the fertilizer industry or fed to a Claus plant for the preparation of elemental sulfur. In addition to addressing the environmental and health problems associated with sulfur dioxide emissions, this approach recovers the sulfur values from coal and other sulfur-containing carbon fuels. However, these gas streams frequently have relatively low sulfur dioxide concentration and a high concentration of water vapor. Where sulfur dioxide concentration in the gas fed to a sulfuric acid plant is less than about 4 to 5 percent by volume, problems may arise with respect to both the water balance and the energy balance in the acid plant. More particularly, the material balance of a conventional sulfuric acid plant requires that the H2O/SO2 molar ratio in the sulfur dioxide-containing gas stream fed to the plant be no higher than the H2O/SO2 molar ratio in the product acid. If the desired product acid concentration is 98.5 percent or above, this ratio cannot be more than about 1.08 in the sulfur dioxide-containing gas stream fed to the plant. As generated, effluent gases from metallurgical processes and flue gases from the combustion of sulfurous fuels often have a water vapor content well above the 1.1 ratio which cannot be sufficiently reduced by cooling the gas without significant capital and energy expenditures. Moreover, if the sulfur dioxide gas strength of the source gas is below about 4 to 5 percent by volume, it may not be sufficient for autothermal operation of the catalytic converter. That is, the heat of conversion of sulfur dioxide to sulfur trioxide may not be great enough to heat the incoming gases to catalyst operating temperature and, as a consequence, heat from some external source must be supplied. This in turn also increases both operating costs and capital requirements for the sulfuric acid facility.
One way of enhancing the sulfur dioxide strength of gaseous effluents is by selectively absorbing the sulfur dioxide in a suitable solvent and subsequently stripping the absorbed sulfur dioxide to produce regenerated solvent and a gas enriched in sulfur dioxide content. A variety of aqueous and organic solvents have been used in regenerative sulfur dioxide absorption/desorption processes. For example, aqueous solutions of alkali metals (e.g., sodium sulfite/bisulfite solution), amines (e.g., alkanolamines, tetrahydroxyethylalkylenediamines, etc.), amine salts and salts of various organic acids have been used as regenerable sulfur dioxide absorbents.
Inorganic aqueous buffer solutions are also effective in absorbing sulfur dioxide. Fung et al. (2000) provides data on the solubility of sulfur dioxide for a solution 1 Molar of phosphoric acid and sodium carbonate in a ratio of about 1.57 Na/PO4 as a function of temperature. Data are for the virgin mixture and the mixture where 1,000 ppm of adipic acid is added to enhance sulfur dioxide solubility. Fung et al. also indicate that when taken to a boiling temperature, 95% and 65% of the sulfur dioxide is removed from the solution. Calculations on the pH of the solution show that the pH changes from 6 to about 3 once that the sulfur dioxide is absorbed. As with organic solvents there is a slight reaction of sulfur dioxide with oxygen forming sulfur trioxide. This reaction is very limited and when Na2CO3 is used it is further inhibited by its reaction with the free radicals formed during oxidation. The sulfur trioxide that is formed leads to the formation of sodium sulfate, which if its removed by crystallization is removed as the sodium sulfate decahydrate (Na2SO4.10H2O) also known as Glauber's salt. This salt can be removed by taking a slipstream and cooling it to force the precipitation of the Glauber's salt that is easily crystallized and removed by a screen, filtration, centrifugation or other solid liquid separation technique.
U.S. Pat. No. 4,133,650 (Gamerdonk et al.) discloses a regenerative process for recovering sulfur dioxide from exhaust gases using a regenerable, aqueous dicarboxylic acid (e.g., phthalic acid, maleic acid, malonic acid and glutaric acid and mixtures thereof) scrubbing solution buffered to a pH of from about 2.8 to 9. The recovered sulfur dioxide can be used in the production of sulfuric acid.
Similarly, U.S. Pat. No. 2,031,802 (Tyrer) suggests using salts of substantially non-volatile acids having a disassociation constant lying between 1×10−2 and 1×10−5 measured at a dilution of 40 liters per gram molecule and a temperature of 25° C. (e.g., lactic acid, glycolic acid, citric acid and ortho-phosphoric acid) in a regenerative process for the recovery of sulfur dioxide from effluent gases.
U.S. Pat. No. 4,366,134 (Korosy) discloses a regenerative flue gas desulfurization process that utilizes an aqueous solution of potassium citrate buffered to a pH of from about 3 to about 9.
Organic solvents used in sulfur dioxide absorption/desorption processes include dimethyl aniline, tetraethylene glycol dimethyl ether and dibutyl butyl phosphonate. Like most solvents, the capacity of organic solvents is enhanced by higher pressures and lower temperatures. The sulfur dioxide gas is then recovered (and the solvent regenerated) by lowering the pressure and/or increasing the temperature. These organic solvents require the use of metallic construction and often require solvent regeneration due to the formation of sulfuric acid and in some cases due to the reaction of the solvent with sulfur trioxide formed by the side-reaction of sulfur dioxide with oxygen during the absorption/desorption process and usually are more expensive than the inorganic absorption media. The significantly large flue gas flow rates emitted from a coal-fired power generation plant, lead to very large equipment size to recover the sulfur dioxide. Organic solvents that require metallic construction generally do not compete well economically with the wet scrubbers that commonly use fiber reinforced plastic (FRP) construction, coated vessels or low cost alloys.
Conventional organic solvents are also hampered by one or more shortcomings with regard to the characteristics desirable in an absorbent used in a sulfur dioxide absorption/desorption cycle. Many of the solvents currently employed have relatively low sulfur dioxide absorption capacity, especially at the sulfur dioxide partial pressures typically encountered in weak sulfur dioxide-containing effluents (e.g., from about 0.1 to about 5 kPa). Conventional organic solvents often absorb substantial quantities of water vapor from the sulfur dioxide-containing effluent resulting in a significant reduction in the sulfur dioxide absorption capacity of the solvent. As a result, the molar flow rates of conventional solvents needed to satisfy the desired sulfur dioxide absorption efficiency is increased. Furthermore, the absorption of large quantities of water vapor in the solvent may lead to excessive corrosion of process equipment used in the sulfur dioxide absorption/desorption process. Moreover, some conventional organic solvents are susceptible to excessive degradation, such as hydrolysis, or other side reactions or decomposition when the solvent is exposed to high temperatures in acidic environments and/or suffer from high volatility, leading to large solvent losses.
Copending and co-assigned U.S. Ser. No. 13/283,671, filed Oct. 28, 2011 describes a sulfur dioxide recovery process that utilizes a buffered aqueous absorption solution comprising certain weak inorganic or organic acids or salts thereof, preferably certain polyprotic carboxylic acids or salts thereof, to selectively absorb sulfur dioxide from the effluent gas. The absorbed sulfur dioxide is subsequently stripped to regenerate the absorption solution and produce a gas enriched in sulfur dioxide content. The sulfur dioxide-enriched gas may be used as part of the feed gas to a contact sulfuric acid plant or to a Claus plant for the preparation of elemental sulfur or can be used for the production of refined sulfur dioxide. The process of U.S. Ser. No. 13/283,671 is particularly useful in producing a sulfur dioxide-enriched gas from effluent gases relatively weak in sulfur dioxide content. The application also describes processes for simultaneous removal of sulfur dioxide and nitrogen oxides (NOx) from effluent gases and recovery of sulfur dioxide. The process utilizes a buffered aqueous absorption solution further including a metal chelate to absorb sulfur dioxide and NO from the gas and subsequently reducing the absorbed NO to form nitrogen.
Although the process of U.S. Ser. No. 13/283,671 operates at high energy efficiency, a need has remained for further economies in the use of energy in regenerative sulfur dioxide recovery processes.